Among ultraviolet light contained in sunlight, the ultraviolet light having a wavelength of 290 nm or less is absorbed by the ozone layer and therefore does not reach the earth surface, but the ultraviolet light of 290 nm to 400 nm reaches the earth surface and has various effects. From the dermatological point of view, it is known that the intermediate wavelength ultraviolet light of 290-320 nm causes the formation of erythema and blisters, acceleration of melanin formation, pigmentation, etc. The long wavelength ultraviolet light of 320-400 nm has the instant effect of darkening the skin immediately after irradiation, and, since its energy reaches the corium, it is said to affect the elastic fibers in the blood vessel walls and the connective tissues. Of these, the intermediate-long wavelength ultraviolet light accelerates aging of the skin and is believed to be a cause of pigmented spots, freckles, wrinkles, etc.
In order to protect the skin from such ultraviolet light, ultraviolet absorbents such as benzotriazole derivatives, benzophenone derivatives, salicylic acid derivatives, paraminobenzoic acid derivatives, cinnamic acid derivatives, and urocanic acid derivatives have been used.
However, these ultraviolet absorbents are generally oil soluble and could not be added to water based products. Recently ultraviolet protection is deemed important not only for bathing in summer and ski areas in winter but also in daily life, and therefore the ultraviolet protection effect is desired even for common skin care cosmetics. Therefore, development of a water soluble ultraviolet absorbent that can be added in a sufficient amount to water based skin care cosmetics is desired.
However, there have been very few water soluble ultraviolet absorbents; sodium 2-hydroxy-4-methoxy-5-sulfoxonium benzophenone is known to be currently used. However, since this substance is a sulfonate, it affects the pH of the formula system and the pH of the formula system affects the ultraviolet absorption region, which is problematic. Also, although this substance is water soluble its solubility is merely approximately 6% at 25° C. and there is a problem in that a high blend ratio in a product leads to precipitation at lower temperatures. Furthermore, this substance also has absorption in the visible light region and thus has a light yellow color and therefore affects the color tone of the product, which is a shortcoming.
Patent Document 1 discloses a p-aminobenzamide derivative having a 2-deoxyhexose residue and Patent Document 2 discloses a cinnamide having a 2-deoxyhexose residue; however, their water solubility is low and insufficient and therefore they are not necessarily satisfactory.
On the other hand, ultraviolet absorbents are used also in areas other than pharmaceuticals and cosmetics; for example, they are added to various materials such as paints, dyes, pigments, various resins, synthetic rubber, films, and fibers for the purpose of adding ultraviolet light absorption capacity to protect the product itself or products coated by the paint or film from ultraviolet light and thus prevent degradation, degeneration and such to maintain the product quality. However, conventional ultraviolet absorbents have a problem in that they are sublimed due to heating during paint baking and/or resin molding, or, even without heating, they are sublimed gradually as time goes on and their effect decreases.
Also, when adding an ultraviolet absorbent to an external preparation, it has to be a compound that does not irritate the skin and is highly safe. It is also important that the compound itself be stable, i.e. not decomposed by exposure to sunlight.    Patent Citation 1: Japanese Patent Laid-Open H10-120698 bulletin    Patent Citation 2: Japanese Patent Laid-Open 2002-363195 bulletin